Base station

ABSTRACT

A base station is provided that performs time-division duplex radio communication with a terminal, using part of a frequency band shared with another system. The base station includes a receiver that receives an uplink signal from the terminal for each frequency band in use, and a guard-time-length setter that sets a length of a guard time. The base station also includes a reception level measurer, which measures a reception level of the uplink signal and a reception level of the guard time having a predetermined length, and a frequency band selector, which selects a frequency band in which the reception level of the guard time is not greater than a first threshold. The base station further includes a control signal generator that generates a control signal, and a transmitter that transmits a downlink signal, including the control signal, to the terminal for the frequency band in use.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.14/808,294, filed Jul. 24, 2015, which claims priority to JapaneseApplication Nos. 2015-012250, filed on Jan. 26, 2015 and 2014-153012,filed on Jul. 28, 2014. The disclosures of these documents, includingthe specifications, drawings, and claims, are incorporated herein byreference in their entirety.

TECHNICAL FIELD

The present invention relates to a base station configured to performradio communication with a terminal.

BACKGROUND ART

Field pickup units (FPUs) are known as apparatuses used in a radiocommunication system for video transmission such as live televisionbroadcasting or emergency broadcasting. Such an FPU is used fortransmitting materials of a broadcasting sector and configured totransmit an uplink (UL) signal of the main content information from of ascene-reporting-side mobile station (terminal) to abroadcast-station-side base station, while transmitting a downlink (DL)signal of feedback information from the broadcast-station-side basestation to the scene-reporting-side mobile station (terminal). The videocaptured by a camera is sent via file transmission in real time andtransmitted from the mobile station to the base station as an UL signaland stored in a storage medium and reproduced. In addition, a feedbacksignal and control signal are transmitted from the mobile station to thebase station as a DL signal.

In such a radio communication system, while the DL signal which is thefeedback information from a broadcast-station-side base station to ascene-reporting-side mobile station is necessary, what is desired mostis faster transmission of UL signals which are the main contentinformation such as video information used in broadcasting. An increasein the transmission rate of UL signals requires a Time-Division Duplex(TDD) frame configuration in which the UL period is longer than the DLperiod.

In the TDD system, a guard time is provided between a UL transmissionperiod and a DL transmission period for the purpose of absorbing delayin radio propagation (see Patent Literature (hereinafter, referred to as“PTL”) 1. The amount of delay decreases as the distance between themobile station and the base station decreases. Meanwhile, in order toincrease the transmission rate, it is favorable to set the guard time tobe as short as possible. For this reason, in the related art, the lengthof guard time is set to be variable so that the guard time becomes shortas the distance between the mobile station and the base stationdecreases.

In addition, FPUs share a frequency band with another system such as atransceiver, so that the presence or absence of interference in thefrequency band in use needs to be always monitored to preventinterference with the other system in FPUs. FPU base stations eachmeasure the reception level (interference amount) of a guard time anddetermines that the other system has started using the frequency band(interference has occurred) when the reception level exceeds apredetermined threshold, and the base station stops using the frequencyband. Note that, the reception level (reception quality) is a receivedsignal strength indicator (RSSI), for example, to be more specific.

CITATION LIST Patent Literature

PTL 1

Japanese Patent Application Laid-Open No. HEI 9-51327

SUMMARY OF INVENTION Technical Problem

In the related art, however, a short guard time is set when the distancebetween the terminal and base station is short, so that a sufficientamount of time for measuring the reception level cannot be secured,which leads to a problem in that frequency sensing accuracydeteriorates.

An object of the present invention is thus to provide a base stationcapable of performing sensing of a frequency with high accuracy bysecuring a sufficient amount of time to measure a reception level of aguard time while avoiding a decrease in the transmission rate as much aspossible, even when the distance between a terminal and the base stationis short.

Solution to Problem

A base station according to an aspect of the present invention is a basestation configured to perform time-division duplex radio communicationwith a terminal, using part of a frequency band shared with anothersystem, the base station including: a receiving section that receives anuplink signal from the terminal for each frequency band in use; areception level measurement section that measures a reception level ofthe uplink signal and a reception level of a guard time for thefrequency band in use; a guard-time-length setting section that sets alength of the guard time based on the reception level of the uplinksignal; a frequency band selection section that selects a frequency bandin which the reception level of the guard time is not greater than afirst threshold; an Ack/Nack generation section that generates anAck/Nack for the frequency hand in use, based on an error detectionresult of the received uplink signal; a control signal generationsection that generates, for the frequency band in use, a control signalincluding information indicating the set length of the guard time,information indicating the selected frequency band, and an Ack/Nack; anda transmission section that transmits a downlink signal including thecontrol signal to the terminal for the frequency band in use, in which,when not generating the control signal in a first frequency band, thecontrol signal generation section adds information including an Ack/Nackfor the first frequency band in the control signal in a second frequencyband that is different from the first frequency band, and thetransmission section does not transmit the down link signal when thecontrol signal generation section does not generate the control signalin the first frequency band.

Advantageous Effects of Invention

According to the present invention, sensing of a frequency can beperformed with high accuracy with a transmission rate almost identicalto the conventional transmission rate, even when the distance between aterminal and a base station is short.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration example of aterminal according to Embodiment 1 of the present invention;

FIG. 2 is a block diagram illustrating a configuration example of a basestation according to Embodiment 1 of the present invention;

FIG. 3 is an example of a frame configuration according to Embodiment 1of the present invention;

FIG. 4 is a configuration example of a control signal according toEmbodiment 1 of the present invention;

FIG. 5 is a flowchart illustrating an operation example of the basestation according to Embodiment 1 of the present invention;

FIG. 6 is a block diagram illustrating a configuration example of a basestation according to Embodiment 2;

FIG. 7 is a diagram illustrating a configuration example of a controlsignal according to Embodiment 2;

FIG. 8 is an example of a frame configuration according to Embodiment 2of the present invention;

FIG. 9 is a flowchart illustrating an operation example of the basestation according to Embodiment 2 of the present invention;

FIG. 10 is an example of a frame configuration according to Embodiment 2of the present invention;

FIG. 11 is a block diagram illustrating a configuration example of abase station according to Embodiment 3 of the present invention;

FIGS. 12A and 12B are each an example of a frame configuration accordingto Embodiment 3 of the present invention;

FIG. 13 is a flowchart illustrating an operation example of the basestation according to Embodiment 3 of the present invention;

FIG. 14 is a block diagram illustrating a configuration example of abase station according to Embodiment 4 of the present invention;

FIG. 15 is a diagram illustrating a configuration example of a controlsignal according to Embodiment 4 of the present invention;

FIGS. 16A and 16B are each a diagram illustrating an example of a frameconfiguration according to Embodiment 4 of the present invention;

FIG. 17 is a flowchart illustrating an operation example of the basestation according to Embodiment 4 of the present invention; and

FIGS. 18A and 18B are each a diagram illustrating an example of a frameconfiguration according to Embodiment 4 of the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a detailed description will be given of embodiments of thepresent invention with reference to the drawings.

Embodiment 1

A radio communication system according to Embodiment 1 includes terminal100 illustrated in FIG. 1 and base station 200 illustrated in FIG. 2.Terminal 100 and base station 200 are each an FPU used in transmissionof materials in a broadcasting sector, for example. More specifically,terminal 100 transmits video information or the like to base station 200as a UL signal, and base station 200 transmits feedback information orthe like to terminal 100 as a DL signal.

To begin with, a description will be given of a configuration example ofterminal 100 according to Embodiment 1 with reference to FIG. 1. FIG. 1is a block diagram illustrating a configuration example of terminal 100according to Embodiment 1.

In FIG. 1, terminal 100 mainly includes radio reception section 101,baseband reception processing section 102, Ack/Nack generation section103, control signal generation section 104, control signal processingsection 105, baseband transmission processing section 106, buffer 107,transmission timing adjustment section 108, and radio transmissionsection 109. Hereinafter, each section of terminal 100 will bedescribed.

Radio reception section 101 performs radio reception processing such asamplification, filtering, and/or the like on a radio signal received viaan antenna. Radio reception section 101 acquires a baseband signal bydown-converting a resultant signal of the radio reception processingusing a frequency band selected by base station 200(frequency-band-selection section 208 to be described, hereinafter)while taking synchronization based on the length of a guard time set bybase station 200 (guard-time-length setting section 209 to be described,hereinafter). Radio reception section 101 outputs the baseband signal tobaseband reception processing section 102.

Baseband reception processing section 102 performs fast Fouriertransform (FFT) processing, demodulation and error correction for orerror detection based on the transmission rate on the baseband signalreceived from radio reception section 101. Baseband reception processingsection 102 outputs information indicating the result of error detectionto Ack/Nack generation section 103.

Moreover, baseband reception processing section 102 performsparallel/serial (P/S) conversion in accordance with the number offrequency bands selected by base station 200 to acquire a control signaland received data. Baseband reception processing section 102 outputs thecontrol signal to control signal processing section 105. Note that,although details will be given hereinafter, this control signal includesinformation indicating the length of a guard time (hereinafter, referredto as “guard-time-length information”), information indicating afrequency band (hereinafter, referred to as “frequency information”),Ack/Nack, and link adaptation information and/or the like.

Ack/Nack generation section 103 generates an Ack/Nack based on theinformation indicating the result of error detection received frombaseband reception processing section 102. Ack/Nack generation section103 outputs an Ack/Nack to control signal generation section 104.

Control signal generation section 104 generates a control signalincluding the Ack/Nack received from Ack/Nack generation section 103 andoutputs the control signal to baseband transmission processing section106.

Control signal processing section 105 outputs the guard-time-lengthinformation included in the control signal received from basebandreception processing section 102 to radio reception section 101,baseband transmission processing section 106, and transmission timingadjustment section 108.

Moreover, control signal processing section 105 outputs the frequencyinformation included in the control signal received from basebandreception processing section 102 to radio reception section 101,baseband reception processing section 102, baseband transmissionprocessing section 106, transmission timing adjustment section 108, andradio transmission section 109.

Control signal processing section 105 indicates new transmission orretransmission to baseband transmission processing section 106 for eachfrequency band based on the Ack/Nack included in the control signalreceived from baseband reception processing section 102.

Furthermore, control signal processing section 105 outputs linkadaptation information included in the control signal received frombaseband reception processing section 102 to baseband receptionprocessing section 102 and baseband transmission processing section 106.

Baseband transmission processing section 106 selects new transmissiondata, or transmission data stored in buffer 107, based on the indicationof control signal processing section 105, forms a UL frame by insertingthe control signal received from control signal generation section 104into the selected transmission data, and performs S/P conversion on theUL frame in accordance with the number of frequency bands selected bybase station 200. When forming a UL frame, baseband transmissionprocessing section 106 adjusts the length of the frame based on thelength of a guard time that is set by base station 200. Basebandtransmission processing section 106 acquires a baseband signal byperforming error correction coding and modulation based on thetransmission rate and inverse fast Fourier transform (IFFT) processingfor each UL frame. Baseband transmission processing section 106 outputsthe baseband signal (UL signal) to transmission timing adjustmentsection 108.

Buffer 107 temporarily stores transmission data.

Transmission timing adjustment section 108 adjusts the transmissiontiming so as to secure the guard time length set by base station 200 forthe signal sequence of the baseband signal received from basebandtransmission processing section 106 and outputs the baseband signal toradio transmission section 109.

Radio transmission section 109 performs radio transmission processingsuch as amplification, filtering, and/or the like on the baseband signalreceived from transmission timing adjustment section 108. Radiotransmission section 109 up-converts a resultant signal of the radiotransmission processing into the frequency band selected by base station200 to acquire a radio signal. Radio transmission section 109 transmitsthe radio signal (UL signal) via an antenna.

The configuration example of terminal 100 according to Embodiment 1 hasbeen described thus far.

Next, a description will be given of a configuration example of basestation 200 according to Embodiment 1 with reference to FIG. 2. FIG. 2is a block diagram illustrating a configuration example of base station200 according to Embodiment 1.

In FIG. 2, base station 200 mainly includes baseband transmissionprocessing section 201, buffer 202, transmission timing adjustmentsection 203, radio transmission section 204, radio reception section205, timer 206, reception level measurement section 207,frequency-band-selection section 208, guard-time-length setting section209, baseband reception processing section 210, control signalprocessing section 211, Ack/Nack generation section 212, transmissionrate setting section 213, and control signal generation section 214.Hereinafter, each section of base station 200 will be described.

Baseband transmission processing section 201 selects new transmissiondata, or transmission data stored in buffer 202, based on the indicationof control signal processing section 211, forms a DL frame by insertingthe control signal generated by control signal generation section 214into the selected transmission data, and performs S/P conversion on theDL frame in accordance with the number of frequency bands selected byfrequency-band-selection station 208. When forming a DL frame, basebandtransmission processing section 201 adjusts the length of a frame basedon the length of a guard time that is set by guard-time-length settingsection 209. Baseband transmission processing section 201 acquires abaseband signal by performing error correction coding and modulationbased on the transmission rate and IFFT processing for each DL frame.Baseband transmission processing section 201 outputs the baseband signal(DL signal) to transmission timing adjustment section 203.

Buffer 202 temporarily stores transmission data.

Transmission timing adjustment section 203 adjusts the transmissiontiming so as to secure the guard time length set by guard-time-lengthsetting section 209 for the signal sequence of the baseband signalreceived from baseband transmission processing section 201 and outputsthe baseband signal to radio transmission section 204.

Radio transmission section 204 performs radio transmission processingsuch as amplification, filtering, and/or the like on the baseband signalreceived from transmission timing adjustment section 203. Radiotransmission section 204 up-converts a resultant signal of the radiotransmission processing into the frequency band selected byfrequency-band-selection section 208 to acquire a radio signal. Radiotransmission section 204 transmits the radio signal (DL signal) via anantenna.

Radio reception section 205 performs radio reception processing such asamplification, filtering, and/or the like on the radio signal receivedvia an antenna. Radio reception section 205 acquires a baseband signalby down-converting a resultant signal of the radio reception processingusing the frequency band selected by frequency-band-selection section208 while taking synchronization based on the length of a guard time setby guard-time-length setting section 209. Radio reception section 205outputs the baseband signal to reception level measurement section 207and baseband reception processing section 210.

Timer 206 is set for a predetermined first period of time (e.g., 10 ms)and a predetermined second period of time (e.g., 10 s), and uponexpiration of the first period of time, timer 206 outputs a firstexpiration signal indicating expiration of the first period of time toreception level measurement section 207, and upon expiration of thesecond period of time, timer 206 outputs a second expiration signalindicating expiration of the second period of time to reception levelmeasurement section 207 and guard-time-length setting section 209.

Upon reception of the first expiration signal from timer 206, receptionlevel measurement section 207 measures the reception level of the ULsignal (received signal). Reception level measurement section 207outputs the information indicating the measured reception level of theUL signal to guard-time-length setting section 209 and transmission ratesetting section 213.

Upon reception of the second expiration signal from timer 206, receptionlevel measurement section 207 measures the reception level of the guardtime of the subsequent frame. Reception level measurement section 207outputs the information indicating the measured reception level of theguard time to frequency-band-selection section 208.

Frequency-band-selection section 208 selects a frequency band in whichthe reception level of the guard time measured by reception levelmeasurement section 207 is equal to or less than a first threshold.Frequency-band-selection section 208 outputs the frequency informationindicating the selected frequency band to radio transmission section204, radio reception section 205, baseband transmission processingsection 201, baseband reception processing section 210, and controlsignal generation section 214.

Guard-time-length setting section 209 sets the guard time length of thesubsequent frame according to whether or not the second expirationsignal has been received from timer 206.

In a case where no second expiration signal has been received from timer206, for example, guard-time-length setting section 209 sets a firstlength for the guard time length of the subsequent frame based on theinformation indicating the reception level of the UL signal receivedfrom reception level measurement section 207.

Meanwhile, in a case where the second expiration signal has beenreceived from timer 206, for example, guard-time-length setting section209 sets a second length (predetermined length required for measurementof reception level) for the guard time length of the subsequent frame.

FIG. 3 illustrates an example of a frame configuration according toEmbodiment 1. Note that, the length of one frame is set to 10 ms in theexample illustrated in FIG. 3.

In FIG. 3, guard time 10 is a guard time that is set with the firstlength, while guard time 11 is a guard time that is set with the secondlength. As illustrated in FIG. 3, even when the distance between theterminal and the base station is short and thus a short guard time issupposed to be set for guard time 10, which is used for the normalperiod, guard time 11, which is longer than guard time 10 used for thenormal period, is set for the frame subsequent to the frame transmittedwhen the second period of time passes. Note that, when the distancebetween the terminal and the base station is long, and guard time 10longer than guard time 11 is set, guard-time-length setting section 209sets the first length (guard time 10) for the length of the guard timeeven during measurement of a reception level.

Guard-time-length setting section 209 outputs the guard-time-lengthinformation indicating the set length of the guard time to basebandtransmission processing section 201, transmission timing adjustmentsection 203, radio reception section 205, and control signal generationsection 214.

Baseband reception processing section 210 performs FFT processing,demodulation and error correction for error detection based on thetransmission rate on the baseband signal received from radio receptionsection 205. Baseband reception processing section 210 outputsinformation indicating the result of error detection to Ack/Nackgeneration section 212.

Moreover, baseband reception processing section 210 performs P/Sconversion in accordance with the number of frequency bands selected byfrequency-band-selection section 208 to acquire a control signal andreceived data. Baseband reception processing section 210 outputs thecontrol signal to control signal processing section 211.

Control signal processing section 211 indicates new transmission orretransmission to baseband transmission processing section 201 for eachfrequency band based on the Ack/Nack included in the control signalreceived from baseband reception processing section 210.

Ack/Nack generation section 212 generates, for each frequency band, anAck/Nack based on the information indicating the result of errordetection received from baseband reception processing section 210.Ack/Nack generation section 212 outputs an Ack/Nack to transmission ratesetting section 213 and control signal generation section 214.

Transmission rate setting section 213 sets a transmission rate for eachfrequency band based on the Ack/Nack received from Ack/Nack generationsection 212 and the information indicating the reception level of the ULsignal received from reception level measurement section 207.Transmission rate setting section 213 outputs link adaptationinformation indicating the modulation scheme corresponding to thetransmission rate and the coding rate for error correction and/or thelike to control signal generation section 214.

Transmission rate setting section 213 indicates the modulation schemeand coding rate and/or the like for the transmission data for eachfrequency band to baseband transmission processing section 201.

In addition, transmission rate setting section 213 indicates thedemodulation scheme and coding rate and/or the like for the basebandsignal for each frequency band to baseband reception processing section210.

Control signal generation section 214 generates a control signal andoutputs the control signal to baseband transmission processing section201. An example of the control signal generated in this processing isillustrated in FIG. 4.

In FIG. 4, the guard-time-length information is the information receivedby control signal generation section 214 from guard-time-length settingsection 209 and indicating the first length or the second length. Thefrequency information is the information received by control signalgeneration section 214 from frequency-band-selection section 208. TheAck/Nack is the information received by control signal generationsection 214 from Ack/Nack generation section 212. The link adaptationinformation is the information received by control signal generationsection 214 from transmission rate setting section 213. Note that, thecontrol signal may include information other than the informationillustrated in FIG. 4.

The configuration example of base station 200 according to Embodiment 1has been described thus far.

Next, a description will be given of an operation example of basestation 200 according to Embodiment 1 with reference to FIG. 5. FIG. 5is a flowchart illustrating the operation example of base station 200according to Embodiment 1.

Timer 206 starts for the second period of time (step S1).

When the second period of time set for timer 206 has not expired (stepS2: NO), i.e., when it is not time to measure the reception level of aguard time (hereinafter, may be referred to as “normal period”),guard-time-length setting section 209 sets the first length for thelength of the guard time for the subsequent frame based on the distancebetween the terminal and the base station (step S3).

Meanwhile, when the second period of time set for timer 206 has expired(step S2: YES), i.e., when it is time to measure the reception level ofa guard time, guard-time-length setting section 209 sets the secondlength, which is a predetermined length required for measurement of areception level, for the length of the guard time for the subsequentframe (step S4). Reception level measurement section 207 measures thereception level of the guard time of the frequency band (step S5).

Control signal generation section 214 generates a control signalincluding the guard-time-length information, frequency information,Ack/Nack, and link adaptation information (step S6).

Radio transmission section 204 transmits a DL signal including thecontrol signal to terminal 100 (step S7).

The operation example of base station 200 according to Embodiment 1 hasbeen described thus far.

As described above, the base station according to Embodiment 1 ischaracterized in that the base station sets a guard time length so as tosecure a predetermined length required for measurement of a receptionlevel when it is time to measure the reception level of the guard time.Thus, even when the distance between the terminal and the base stationis short, sensing of a frequency can be performed with high accuracywithout a decrease in the transmission efficiency.

Note that, when the length of one frame is 10 ms, and the interval formeasuring the reception levels of guard times is 10 s, a long guard timeneeds to be set only once in every one thousand frames, so that thetransmission rate according to Embodiment 1 is almost the same as thatof the related art. Embodiment 1 has been described with the case wherethe length of one frame is 10 ms, and the interval for measuring thereception levels of guard times is 10 s, as an example, but the presentinvention is not limited to this case, and any value can be set for thelength of one frame as well as the interval for measuring the receptionlevels of guard times.

Embodiment 2

A description will be given of Embodiment 2. The configuration ofterminal 100 according to Embodiment 2 is the same as the configurationdescribed in FIG. 1, which is described in Embodiment 1, so that thesame description will not be repeated, hereinafter.

A description will be given of a configuration example of base station200A according to Embodiment 2 with reference to FIG. 6. FIG. 6 is ablock diagram illustrating the configuration example of base station200A according to Embodiment 2. Each section of base station 200Aillustrated in FIG. 6 is assigned the same reference numeral assigned tothe corresponding section of base station 200 illustrated in FIG. 2.Hereinafter, a description will be given of only a configuration elementthat performs an operation different from the operations in FIG. 2,among the sections illustrated in FIG. 6.

A predetermined first period of time (e.g., 10 ms) and a predeterminedsecond period of time (e.g., 10 s) are set for timer 206, and uponexpiration of the first period of time, timer 206 outputs a firstexpiration signal indicating expiration of the first period of time toreception level measurement section 207, and upon expiration of thesecond period of time, timer 206 outputs a second expiration signalindicating expiration of the second period of time to basebandtransmission processing section 201, reception level measurement section207, and control signal generation section 214.

Baseband transmission processing section 201 operates as follows,according to whether or not the second expiration signal has beenreceived from timer 206 (i.e., whether or not it is time to measure thereception level of a guard time).

In a case where no second expiration signal has been received from timer206 (i.e., it is not time to measure the reception level of a guardtime), a DL signal is generated by inserting the control signalgenerated by control signal generation section 214 into transmissiondata (new transmission or retransmission) to form a DL in all thefrequency bands in use. The operation to be performed after the DLsignal is generated is the same as that of Embodiment 1, so that thesame description will not be repeated hereinafter.

Meanwhile, in a case where a second expiration signal has been receivedfrom timer 206 (i.e., it is time to measure the reception level of aguard time), baseband transmission processing section 201 does notgenerate a DL signal for the subsequent frame in a frequency band (firstfrequency band) that is a measurement target for the reception level ofa guard time (hereinafter, referred to as “measurement target”).

Reception level measurement section 207 measures the reception level ofa guard time of a received signal in each frequency band every timereceiving the second expiration signal from timer 206. Reception levelmeasurement section 207 outputs the information indicating the measuredreception level of the guard time to frequency-hand-selection section208.

Control signal generation section 214 operates as follows, according towhether or not the second expiration signal has been received from timer206 (i.e., whether or not it is time to measure the reception level of aguard time).

In a case where no second expiration signal has been received from timer206 (i.e., it is not time to measure the reception level of a guardtime), control signal generation section 214 generates, in thesubsequent frame, a control signal in all the frequency bands in use asin Embodiment 1 and outputs the control signal to baseband transmissionprocessing section 201.

Meanwhile, in a case where a second expiration signal has been receivedfrom timer 206 (i.e., it is time to measure the reception level of aguard time), control signal generation section 214 generates a controlsignal in the subsequent frame in all the frequency bands except thefrequency band of the measurement target and outputs the control signalto baseband transmission processing section 201. However, theguard-time-length information, frequency information, Ack/Nack, and linkadaptation information of the frequency band of the measurement target(first frequency band) are included in a control signal of anotherfrequency band (second frequency band) (e.g., a frequency band havingthe highest reception level). FIG. 7 illustrates an example of thiscontrol signal.

As illustrated in FIG. 7, the guard-time-length information, frequencyinformation, Ack/Nack, and link adaptation information of frequency bandf1 of the measurement target are included in the control signal offrequency band f2, which is different from first frequency band f1. Uponreception of the second expiration signal from timer 206, basebandtransmission processing section 201 forms a DL frame by inserting thecontrol signal illustrated in FIG. 7 into transmission data in thesubsequent frame.

FIG. 8 illustrates an example of a frame configuration according toEmbodiment 2. As illustrated in FIG. 8, no DL signal is transmitted inthe frame subsequent to the frame transmitted when the second period oftime passes in frequency band f1, which is the measurement target, andthe portion of the frame where no DL signal is transmitted becomes guardtime 20. Thus, a guard time longer than guard time 10, which is used forthe normal period, can be secured. Meanwhile, in frequency band f2,which is different from frequency band f1, DL signal 21 is transmittedin the portion of the frame corresponding to guard time 20.

Note that, when the measurement target frequency band is f2, and anotherfrequency band is f1, no DL signal is transmitted in the framesubsequent to the frame transmitted when the second period of timepasses in frequency band f2 of the measurement target, and the portionof the frame where no DL signal is transmitted becomes guard time 22.Meanwhile, in frequency band f1, which is another frequency band, DLsignal 23 in the portion of the frame corresponding to guard time 22 istransmitted. Note that, the guard-time-length information, frequencyinformation, Ack/Nack, and link adaptation information in frequency bandf2, which is the measurement target, are included in the control signalof DL signal 23.

The configuration example of base station 200A according to Embodiment 2has been described thus far.

Next, a description will be given of an operation example of basestation 200A according to Embodiment 2 with reference to FIG. 9. FIG. 9is a flowchart illustrating an operation example of base station 200Aaccording to Embodiment 2.

Timer 206 starts for a predetermined second period of time (step S11).

When the second period of time set for timer 206 has not expired (stepS12: NO), i.e., when it is not time to measure the reception level of aguard time (during the normal period), baseband transmission processingsection 201 generates a DL signal including a control signal (step S13),and radio transmission section 204 transmits the DL signal including thecontrol signal to terminal 100 (step S14).

Meanwhile, when the second period of time set for timer 206 has expired(step S12: YES), i.e., when it is time to measure the reception level ofa guard time, baseband transmission processing section 201 stopsgenerating a DL signal in the frequency band (step S15). Reception levelmeasurement section 207 measures the reception level of the guard timeof the frequency band (step S16).

The operation example of base station 200A according to Embodiment 2 hasbeen described thus far.

As described above, the base station according to Embodiment 2 ischaracterized in that the base station secures the period normallyallocated for transmission of a DL frame, as a guard time when it istime to measure the reception level of a guard time, and includesinformation related to the measurement target frequency band (firstfrequency band) in a control signal of another frequency band (secondfrequency band). Thus, even when the distance between the terminal andbase station is short, sensing of a frequency can be performed with highaccuracy without a decrease in the transmission efficiency.

In Embodiment 2, in a frame in which a DL signal is transmitted in acertain frequency band (second frequency band), a UL signal may betransmitted in another frequency band (first frequency band) in the sameframe during a period other than a period in which measurement of areception level is performed. An example of the frame configuration inthis case is illustrated in FIG. 10. As illustrated in FIG. 10, when aDL signal is transmitted in frequency band f1 during a period when it isnot time to measure the reception level of a guard time (during thenormal period), a UL signal is transmitted in frequency band f2 in theportion of the frame corresponding to the portion of the frame where theDL signal is transmitted in frequency band f1. Note that, it is possibleto alternately use frequency bands f1 and f2 for transmitting a DLsignal as illustrated in FIG. 10 or to use any one of frequency bands f1and f2 except for the period when measurement of a reception level isperformed.

Embodiment 3

In Embodiment 1, a description has been given of the case whereguard-time-length setting section 209 sets the second length(predetermined length required for measurement of a reception level) forthe length of a guard time always at the interval of the second periodsof time.

In Embodiment 3, a description will be given of a case where thecommunication quality is estimated in advance, and only when thecommunication quality deteriorates, guard-time-length setting section209 sets a second length for the length of the guard time at the timingwhen the second period of time passes. Note that, the configuration ofterminal 100 according to Embodiment 3 is the same as the configurationillustrated in FIG. 1, which is described in Embodiment 1, so that thedescription of the configuration will not be repeated, hereinafter.

A description will be given of a configuration example of base station200B according to Embodiment 3 with reference to FIG. 11. FIG. 11 is ablock diagram illustrating the configuration example of base station200B according to Embodiment 3. Note that, in base station 200Billustrated in FIG. 11, the sections which are common to the sections ofbase station 200 illustrated in FIG. 2 are assigned the same referencenumerals and the detailed descriptions of the sections will not berepeated, hereinafter. Base station 200B illustrated in FIG. 11 employsa configuration which is different from base station 200 illustrated inFIG. 2 in that quality estimation section 215 is added.

Upon reception of the first expiration signal from timer 206, receptionlevel measurement section 207 measures the reception level of a ULsignal (received signal). Reception level measurement section 207outputs the information indicating the measured reception level of theUL signal to guard-time-length setting section 209, transmission ratesetting section 213, and quality estimation section 215.

Upon reception of the second expiration signal from timer 206, receptionlevel measurement section 207 measures the reception level of a guardtime in the subsequent frame only when the signal outputted from qualityestimation section 215 indicates that the communication quality hasdeteriorated. Reception level measurement section 207 outputs theinformation indicating the measured reception level of the guard time tofrequency-band-selection section 208.

Quality estimation section 215 makes a comparison of a magnituderelationship between the reception level of the UL signal measured byreception level measurement section 207 with a predetermined secondthreshold. Quality estimation section 215 estimates that thecommunication quality is favorable when the reception level is greaterthan the second threshold or estimates that the communication qualityhas deteriorated when the reception level is not greater than the secondthreshold. Quality estimation section 215 outputs a signal indicating aresult of the estimation to reception level measurement section 207 andguard-time-length setting section 209.

Upon reception of the second expiration signal from timer 206,guard-time-length setting section 209 sets a second length for thelength of a guard time in the subsequent frame when the signal outputtedfrom quality estimation section 215 indicates that the communicationquality has deteriorated, or guard-time-length setting section 209 setsa first length for the length of a guard time in the subsequent framewhen the signal outputted from quality estimation section 215 indicatesthat the communication quality is favorable based on the informationindicating the reception level of the UL signal received from receptionlevel measurement section 207.

FIGS. 12A and 12B each illustrate an example of a frame configurationaccording to Embodiment 3. FIG. 12A illustrates a case where thecommunication quality has deteriorated, while FIG. 12B illustrates acase where the communication quality is favorable. Note that, anassumption is made that the frame length is 10 ms in the examples ofFIGS. 12A and 12B.

In FIGS. 12A and 12B, guard time 10 is a guard time that is set with thefirst length, while guard time 11 is a guard time that is set with thesecond length. As illustrated in FIG. 12A, in a case where thecommunication quality has deteriorated, guard time 11, which is longerthan guard time 10 used for the normal period, is set in the framesubsequent to the frame transmitted when the second period of timepasses. Meanwhile, as illustrated in FIG. 12B, in a case where thecommunication quality is favorable, guard time 10 used for the normalperiod is set in the frame subsequent to the frame transmitted when thesecond period of time passes. Thus, it is made possible to increase thetransmission period for the UL signal, thus making it possible toincrease the transmission rate.

Note that, guard-time-length setting section 209 performs the sameoperation as that in Embodiment 1 in a case where no second expirationsignal has been received from timer 206.

The configuration example of base station 200B according to Embodiment 3has been described thus far.

Next, a description will be given of an operation example of basestation 200B according to Embodiment 3 with reference to FIG. 13. FIG.13 is a flowchart illustrating the operation example of base station200B according to Embodiment 3. Note that, in the flowchart illustratedin FIG. 13, the steps which are common to the flowchart illustrated in

FIG. 5 are assigned the same reference numerals, and the detaileddescriptions of the steps will not be repeated, hereinafter. Theflowchart illustrated in FIG. 13 is different from the flowchartillustrated in FIG. 5 in that step S21 is added between steps S2 and S4.

In a case where the second period of time set for timer 206 has expired(step S2: YES) and the communication quality has deteriorated (step S21:YES), guard-time-length setting section 209 sets the second length forthe length of a guard time of the subsequent frame (step S4). Meanwhile,in a case where the second period of time set for timer 206 has expired(step S2: YES) and the communication quality is favorable (step S21:NO), guard-time-length setting section 209 sets the first length for thelength of a guard time of the subsequent frame (step S3).

The operation example of base station 200B according to Embodiment 3 hasbeen described thus far.

As described above, the base station according to Embodiment 3 ischaracterized in that the base station sets the length of a guard timeso as to secure a predetermined length required for measurement of thereception level in a case where it is time to measure the receptionlevel of a guard time and the communication quality has deteriorated.Thus, in addition to the effects obtained in Embodiment 1, it ispossible to achieve a further increase in the transmission rate ascompared with Embodiment 1.

Embodiment 4

In Embodiment 2, a description has been given of the case where controlsignal generation section 214 stops generating a control signal andbaseband transmission processing section 201 stops generating a frameand provides a guard time always at the interval of the second periodsof time.

In Embodiment 4, a description will be given of a case where thecommunication quality is estimated in advance, and only when thecommunication quality has deteriorated, control signal generationsection 214 stops generating a control signal and baseband transmissionprocessing section 201 stops generating a frame and provides a guardtime at the time when the second period of time passes. In Embodiment 4,the configuration of terminal 100 is the same as the configurationillustrated in FIG. 1, which is described in Embodiment 1, so that thedescription of the configuration will not be repeated, hereinafter.

A description will be given of a configuration example of base station200C according to Embodiment 4 with reference to FIG. 14. FIG. 14 is ablock diagram illustrating the configuration example of base station200C according to Embodiment 4. Note that, in base station 200Cillustrated in FIG. 14, the sections which are common to the sections ofbase station 200A illustrated in FIG. 6 are assigned the same referencenumerals and the detailed descriptions of the sections will not berepeated, hereinafter. Base station 200C illustrated in FIG. 14 employsa configuration which is different from base station 200A illustrated inFIG. 6 in that quality estimation section 215 is added.

Upon reception of the first expiration signal from timer 206, receptionlevel measurement section 207 measures the reception level of a ULsignal (received signal). Reception level measurement section 207outputs the information indicating the measured reception level of theUL signal to guard-time-length setting section 209, transmission ratesetting section 213, and quality estimation section 215.

In addition, upon reception of the second expiration signal from timer206, reception level measurement section 207 measures the receptionlevel of a guard time only when the signal outputted from qualityestimation section 215 indicates that the communication quality hasdeteriorated. Reception level measurement section 207 outputs theinformation indicating the measured reception level of the guard time tofrequency-band-selection section 208.

Quality estimation section 215 makes a comparison of a magnituderelationship between the reception level of the UL signal measured byreception level measurement section 207 with a predetermined secondthreshold. Quality estimation section 215 estimates that thecommunication quality is favorable when the reception level is greaterthan the second threshold or estimates that the communication qualityhas deteriorated when the reception level is not greater than the secondthreshold. Quality estimation section 215 outputs a signal indicating aresult of the estimation to reception level measurement section 207 andcontrol signal generation section 214.

Upon reception of the second expiration signal from timer 206, controlsignal generation section 214 generates a control signal illustrated inFIG. 15 in the measurement target frequency band (first frequency band)of the current frame and outputs the control signal to basebandtransmission processing section 201. The control signal illustrated inFIG. 15 is different from the control signal illustrated in FIG. 4(guard-time-length information, frequency information, Ack/Nack, andlink adaptation information) in that quality information which is asignal indicating an estimation result of quality estimation section 215is added.

Control signal generation section 214 performs the same operation asthat in Embodiment 2 in a frame other than the current frame transmittedwhen control signal generation section 214 receives the secondexpiration signal from timer 206.

Baseband transmission processing section 201 forms DL frames (DL31 andDL32 in FIGS. 16A and 16B) by inserting the control signal illustratedin FIG. 15 into transmission data in the current frame transmitted whenthe second period of time passes.

In a case where the quality information indicates that the communicationquality is favorable, no guard time needs to be provided, so thatterminal 100 transmits a UL signal in the portion of the frame where aguard time is supposed to be transmitted.

FIGS. 16A and 16B each illustrate an example of a frame configurationaccording to Embodiment 4. FIG. 16A illustrates a case where thecommunication quality has deteriorated, while FIG. 16B illustrates acase where the communication quality is favorable. Note that, anassumption is made that the length of one frame in the examples of FIGS.16A and 16B is 10 ms.

As illustrated in FIG. 16A, in a case where the communication qualityhas deteriorated, no DL signal is transmitted in frequency band f1(f2)of the measurement target in the frame subsequent to the frametransmitted when the second period of time passes as in the case ofEmbodiment 2 (FIG. 8), and the portion of the frame where a DL signalwould have been transmitted otherwise becomes guard time 20(22). Thus, aguard time longer than guard time 10 used for the normal period can besecured. Note that, in frequency band f2(f1) which is different from themeasurement target frequency band, DL signal 21(23) is transmitted usingthe portion of the frame corresponding to guard time 20(22).

Meanwhile, as illustrated in FIG. 16B, in a case where the communicationquality is favorable, no DL signal is transmitted in frequency bandf1(f2) of the measurement target in the frame subsequent to the frametransmitted when the second period of time passes, and a UL signal istransmitted using the portion of the frame where a DL signal is supposedto be transmitted. Thus, it is made possible to increase thetransmission period for the UL signal, thus making it possible toincrease the transmission rate. Note that, DL signal 21(23) istransmitted in frequency band f2(f1) which is different from themeasurement target frequency band.

The configuration example of base station 200C according to Embodiment 4has been described thus far.

Next, a description will be given of an operation example of basestation 200C according to Embodiment 4 with reference to FIG. 17. FIG.17 is a flowchart illustrating the operation example of base station200C according to Embodiment 4. Note that, in the flowchart illustratedin FIG. 17, the steps which are common to the flowchart illustrated inFIG. 9 are assigned the same reference numerals, and the detaileddescriptions of the steps will not be repeated, hereinafter. Theflowchart illustrated in FIG. 17 is different from the flowchartillustrated in FIG. 9 in that step S31 is added between steps S15 andS16 of the flow illustrated in FIG. 9 and that step S32 is added as adestination when a determination result in step S31 is NO.

In a case where the second period of time set for timer 206 has expired(step S12: YES), baseband transmission processing section 201 stopsgenerating a DL signal in the frequency band (step S15). Meanwhile, in acase where the communication quality has deteriorated (step S31: YES),reception level measurement section 207 measures the reception level ofthe guard time of the frequency band (step S16).

Meanwhile, in a case where the communication quality is favorable (stepS31: NO), base station 200C receives a UL signal (step S32). In thiscase, reception level measurement section 207 does not measure thereception level of the guard time of the frequency band.

The operation example of base station 200C according to Embodiment 4 hasbeen described thus far.

As described above, the base station according to Embodiment 4 ischaracterized in that the base station secures, as a guard time, theperiod normally allocated for transmission of a DL frame, and includesinformation related to the measurement target frequency hand (firstfrequency band) in a control signal of another frequency band (secondfrequency band) in a case where it is time to measure the receptionlevel of a guard time and the communication quality has deteriorated.Thus, in addition to the effects obtained in Embodiment 2, it ispossible to achieve a further increase in the transmission rate ascompared with Embodiment 2.

In Embodiment 4, in a frame in which a DL signal is transmitted in acertain frequency band (second frequency band), a UL signal may betransmitted in another frequency band (first frequency band) during aperiod other than a period in which measurement of the reception levelis performed. An example of the frame configuration in this case isillustrated in FIGS. 18A and 18B. FIG. 18A illustrates a case where thecommunication quality has deteriorated, while FIG. 18B illustrates acase where the communication quality is favorable. As illustrated inFIGS. 18A and 18B, when a DL signal is transmitted in frequency band f1during a period when it is not time to measure the reception level of aguard time (during the normal period), a UL signal is transmitted infrequency band f2 in the portion of the frame corresponding to theportion of the frame where the DL signal is transmitted in frequencyband 1.

Embodiments of the present invention have been described thus far. Thepresent invention is, however, not limited to these embodiments, andvarious modifications are possible.

For example, the same or mutually different modulation scheme to be usedand/or error correction coding rate may be set for the frequency bands.

In the present invention, depending on the communication state orapplication or the like, it is possible to fixedly use the frameconfiguration illustrated in FIG. 16B or the frame configurationillustrated in FIG. 18B (configuration in which a control signal istransmitted from a frequency band other than the measurement targetfrequency band during a specific period such as a period when frequencysensing is performed). Even when the level of an interference wave ishigh, if the frequency bandwidth used by the interference wave isnarrow, deterioration in characteristics due to the interference occursonly on partial data as long as the communication is multi-carriercommunication such as OFDM. Thus, the deterioration in characteristicsdue to the interference wave can be reduced by frequency interleavingand error correction and/or the like. Accordingly, fixedly using theframe configuration illustrated in FIG. 16B or the frame configurationillustrated in FIG. 18B makes it possible to further improve thetransmission efficiency depending on the communication state orapplication or the like.

INDUSTRIAL APPLICABILITY

The base station according to the present invention is applicable to abase station configured to perform radio communication with a terminal,for example.

REFERENCE SIGNS LIST

-   100 Terminal-   101 Radio reception section-   102 Baseband reception processing section-   103 Ack/Nack generation section-   104 Control signal generation section-   105 Control signal processing section-   106 Baseband transmission processing section-   107 Buffer-   108 Transmission timing adjustment section-   109 Radio transmission section-   200 Base station-   201 Baseband transmission processing section-   202 Buffer-   203 Transmission timing adjustment section-   204 Radio transmission section-   205 Radio reception section-   206 Timer-   207 Reception level measurement section-   208 Frequency-band-selection section-   209 Guard-time-length setting section-   210 Baseband reception processing section-   211 Control signal processing section-   212 Ack/Nack generation section-   213 Transmission rate setting section-   214 Control signal generation section-   215 Quality estimation section

What is claimed is:
 1. A base station configured to performtime-division duplex radio communication with a terminal, using part ofa frequency band shared with another system, the base stationcomprising: a receiver that receives an uplink signal from the terminalfor each frequency band in use; a guard-time-length setter that sets alength of a guard time; a reception level measurer that measures, forthe frequency band in use, a reception level of the uplink signal and areception level of the guard time having a predetermined length; afrequency band selector that selects a frequency band in which thereception level of the guard time is not greater than a first threshold;a control signal generator that generates, for the frequency band inuse, a control signal including information indicating the set length ofthe guard time, and information indicating the selected frequency band;and a transmitter that transmits a downlink signal including the controlsignal to the terminal for the frequency band in use, wherein theguard-time-length setter sets a first length for the guard time based onthe reception level of the uplink signal during a period until a timerthat is set for a predetermined period of time expires, and theguard-time-length setter sets a second length for the guard time whenthe timer that is set for the predetermined period of time has expired,the second length being longer than the first length.
 2. The basestation according to claim 1, further comprising: a quality estimatorthat estimates channel quality by comparing the reception level of theuplink signal with a second threshold, wherein the guard-time-lengthsetter sets the first length for the guard time in a case where thereception level of the uplink signal is not greater than the secondthreshold, even when the timer that is set for the predetermined periodof time has expired.